Notch receptors control normal cell growth, differentiation, and death in multicellular organisms through a signaling pathway that is triggered by ligand-induced proteolysis (Bray, Nat. Rev. Mol. Cell Biol. 7(9):678-689, 2006). The mature Notch heterodimer after furin-like protease cleavage at site S1 is held in an auto-inhibited state by a juxtamembrane negative regulatory region (NRR) consisting of three Lin12/Notch repeats (LNR-A, B, C) and the heterodimerization (HD) domain. The HD domain is divided into N-terminal (HD1) and C-terminal (HD2) halves by cleavage at site S1. Through an uncertain mechanism, binding of ligands of the Delta/Serrate/Lag-2 (DSL) family to the N-terminal, EGF-repeat region relieves this inhibition and induces two successive additional cleavages at S2 near the C-terminal region HD-2, and S3 within transmembrane domain in Notch that are catalyzed by ADAM-type metalloproteinase and gamma-Secretase, respectively (Gordon, W. R., et. al, Nature Structural & Molecular Biology, 2007, volume 14, 295-300). The latter cleavage releases the intracellular domain of Notch (NotchICD), permitting it to translocate to the nucleus and activate the transcription of target genes.
In mammalian cells, there are four known Notch receptors. Notch1-4 have broad, overlapping patterns of expression in embryonic and adult tissues, and fulfill non-redundant roles during hematopoietic stem cell specification, T cell development, intestinal crypt cell specification and vascular development. Acquired abnormalities involving specific Notch1 receptors have been implicated in cancers, such as T cell acute lymphoblastic leukemia (T-ALL), breast cancer and lung cancer. In addition, activated Notch1 is a potent inducer of leukemia in murine models and is over-expressed in various solid tumors, including non-small cell lung cancer, breast cancer and ovarian cancer.
Over 50% of T-ALL patients harbor mutations in the Notch1 receptor some of which result in constitutive cleavage of the receptor and production of the Notch1ICD due in part to Notch1 ligand-hypersenstivity or ligand-independent activation caused by alterations in or near the NRR auto-inhibitory domain. These mutations are categorized into 3 major classes. Class 1 mutations are single amino acid substitutions and small in-frame deletions or insertions in HD1. Class 2 mutations are longer insertions in the distal region of HD2 that relocate the S2-metalloprotease cleavage site beyond the auto-inhibitory NRR domain. Class 3 mutations, also called juxtamembrane expansion (JMEs) mutations, occur from large insertions that displace the NRR away from the cell membrane.
Several strategies are in development to inhibit Notch signaling for therapeutic purposes in cancer. One approach is to block the proteolytic release of intracellular Notch from the membrane by treatment with inhibitors of gamma-secretase (GSIs). Although GSIs have progressed into the clinic, they cannot distinguish individual Notch receptors and cause intestinal toxicity attributed to the inhibition of both Notch1 and Notch2. There is still a need in the art for novel anti-Notch1 therapies for the treatment of cancer while providing reduced side effects, in particular, intestinal toxicity.